A travel distance of a movable object is used together with a traveling direction, for estimating a traveling trajectory, for example. If a traveling trajectory can be estimated, a relative position, which is relative to a position at a certain point of time, can be determined. Even when a radio wave for positioning transmitted by a satellite positioning system including a GPS or the like cannot be received, it is possible to estimate a current position based the past measured position and a traveling trajectory.
A travel distance can be calculated by the product of a velocity and a time. As a method of estimating a velocity of a movable object, Patent Literature 1 discloses a technology of: measuring an acceleration and then calculating a velocity from the acceleration; and additionally calculating a velocity also from a Doppler shift amount of a GPS carrier wave (hereunder a Doppler velocity). In Patent Literature 1, a velocity calculated from an acceleration is corrected successively by a Doppler velocity.
A velocity calculated from an acceleration can be renewed in a short cycle but is more likely to cause an error than a Doppler velocity. Consequently, a velocity calculated from an acceleration is corrected successively by a Doppler velocity. A velocity of good accuracy can be obtained by the correction.
An advantage of the technology according to Patent Literature 1 is that wiring for obtaining a signal of a vehicle velocity sensor is unnecessary when the technology is applied to a vehicle.
Patent Literature 2 discloses a technology of estimating a velocity vector of a vehicle with a high degree of accuracy. Concretely, in the technology according to Patent Literature 2, an expression showing a relationship among a satellite direction vehicle velocity, a tire wheel velocity, a line-of-sight vector from a vehicle to a satellite in a positioning system, a vehicle orientation, and a clock drift is used for estimating a velocity vector. The expression is an expression formed by modifying an expression showing a relationship among a satellite direction velocity, a line-of-sight vector, a velocity vector, and a clock drift by using a constraint condition. The constraint condition comprises the condition that a velocity vector is constrained by a velocity and a yaw rate of a movable object and the condition that the time variation of a clock drift is linear.